diff options
Diffstat (limited to 'manual/mbyte.texi')
| -rw-r--r-- | manual/mbyte.texi | 696 |
1 files changed, 0 insertions, 696 deletions
diff --git a/manual/mbyte.texi b/manual/mbyte.texi deleted file mode 100644 index 8f3c192..0000000 --- a/manual/mbyte.texi +++ /dev/null @@ -1,696 +0,0 @@ -@node Extended Characters, Locales, String and Array Utilities, Top -@c %MENU% Support for extended character sets -@chapter Extended Characters - -A number of languages use character sets that are larger than the range -of values of type @code{char}. Japanese and Chinese are probably the -most familiar examples. - -The GNU C library includes support for two mechanisms for dealing with -extended character sets: multibyte characters and wide characters. This -chapter describes how to use these mechanisms, and the functions for -converting between them. -@cindex extended character sets - -The behavior of the functions in this chapter is affected by the current -locale for character classification---the @code{LC_CTYPE} category; see -@ref{Locale Categories}. This choice of locale selects which multibyte -code is used, and also controls the meanings and characteristics of wide -character codes. - -@menu -* Extended Char Intro:: Multibyte codes versus wide characters. -* Locales and Extended Chars:: The locale selects the character codes. -* Multibyte Char Intro:: How multibyte codes are represented. -* Wide Char Intro:: How wide characters are represented. -* Wide String Conversion:: Converting wide strings to multibyte code - and vice versa. -* Length of Char:: how many bytes make up one multibyte char. -* Converting One Char:: Converting a string character by character. -* Example of Conversion:: Example showing why converting - one character at a time may be useful. -* Shift State:: Multibyte codes with "shift characters". -@end menu - -@node Extended Char Intro, Locales and Extended Chars, , Extended Characters -@section Introduction to Extended Characters - -You can represent extended characters in either of two ways: - -@itemize @bullet -@item -As @dfn{multibyte characters} which can be embedded in an ordinary -string, an array of @code{char} objects. Their advantage is that many -programs and operating systems can handle occasional multibyte -characters scattered among ordinary ASCII characters, without any -change. - -@item -@cindex wide characters -As @dfn{wide characters}, which are like ordinary characters except that -they occupy more bits. The wide character data type, @code{wchar_t}, -has a range large enough to hold extended character codes as well as -old-fashioned ASCII codes. - -An advantage of wide characters is that each character is a single data -object, just like ordinary ASCII characters. There are a few -disadvantages: - -@itemize @bullet -@item -Each existing program must be modified and recompiled to make it use -wide characters. - -@item -Files of wide characters cannot be read by programs that expect ordinary -characters. -@end itemize -@end itemize - -Typically, you use the multibyte character representation as part of the -external program interface, such as reading or writing text to files. -However, it's usually easier to perform internal manipulations on -strings containing extended characters on arrays of @code{wchar_t} -objects, since the uniform representation makes most editing operations -easier. If you do use multibyte characters for files and wide -characters for internal operations, you need to convert between them -when you read and write data. - -If your system supports extended characters, then it supports them both -as multibyte characters and as wide characters. The library includes -functions you can use to convert between the two representations. -These functions are described in this chapter. - -@node Locales and Extended Chars, Multibyte Char Intro, Extended Char Intro, Extended Characters -@section Locales and Extended Characters - -A computer system can support more than one multibyte character code, -and more than one wide character code. The user controls the choice of -codes through the current locale for character classification -(@pxref{Locales}). Each locale specifies a particular multibyte -character code and a particular wide character code. The choice of locale -influences the behavior of the conversion functions in the library. - -Some locales support neither wide characters nor nontrivial multibyte -characters. In these locales, the library conversion functions still -work, even though what they do is basically trivial. - -If you select a new locale for character classification, the internal -shift state maintained by these functions can become confused, so it's -not a good idea to change the locale while you are in the middle of -processing a string. - -@node Multibyte Char Intro, Wide Char Intro, Locales and Extended Chars, Extended Characters -@section Multibyte Characters -@cindex multibyte characters - -In the ordinary ASCII code, a sequence of characters is a sequence of -bytes, and each character is one byte. This is very simple, but -allows for only 256 distinct characters. - -In a @dfn{multibyte character code}, a sequence of characters is a -sequence of bytes, but each character may occupy one or more consecutive -bytes of the sequence. - -@cindex basic byte sequence -There are many different ways of designing a multibyte character code; -different systems use different codes. To specify a particular code -means designating the @dfn{basic} byte sequences---those which represent -a single character---and what characters they stand for. A code that a -computer can actually use must have a finite number of these basic -sequences, and typically none of them is more than a few characters -long. - -These sequences need not all have the same length. In fact, many of -them are just one byte long. Because the basic ASCII characters in the -range from @code{0} to @code{0177} are so important, they stand for -themselves in all multibyte character codes. That is to say, a byte -whose value is @code{0} through @code{0177} is always a character in -itself. The characters which are more than one byte must always start -with a byte in the range from @code{0200} through @code{0377}. - -The byte value @code{0} can be used to terminate a string, just as it is -often used in a string of ASCII characters. - -Specifying the basic byte sequences that represent single characters -automatically gives meanings to many longer byte sequences, as more than -one character. For example, if the two byte sequence @code{0205 049} -stands for the Greek letter alpha, then @code{0205 049 065} must stand -for an alpha followed by an @samp{A} (ASCII code 065), and @code{0205 049 -0205 049} must stand for two alphas in a row. - -If any byte sequence can have more than one meaning as a sequence of -characters, then the multibyte code is ambiguous---and no good. The -codes that systems actually use are all unambiguous. - -In most codes, there are certain sequences of bytes that have no meaning -as a character or characters. These are called @dfn{invalid}. - -The simplest possible multibyte code is a trivial one: - -@quotation -The basic sequences consist of single bytes. -@end quotation - -This particular code is equivalent to not using multibyte characters at -all. It has no invalid sequences. But it can handle only 256 different -characters. - -Here is another possible code which can handle 9376 different -characters: - -@quotation -The basic sequences consist of - -@itemize @bullet -@item -single bytes with values in the range @code{0} through @code{0237}. - -@item -two-byte sequences, in which both of the bytes have values in the range -from @code{0240} through @code{0377}. -@end itemize -@end quotation - -@noindent -This code or a similar one is used on some systems to represent Japanese -characters. The invalid sequences are those which consist of an odd -number of consecutive bytes in the range from @code{0240} through -@code{0377}. - -Here is another multibyte code which can handle more distinct extended -characters---in fact, almost thirty million: - -@quotation -The basic sequences consist of - -@itemize @bullet -@item -single bytes with values in the range @code{0} through @code{0177}. - -@item -sequences of up to four bytes in which the first byte is in the range -from @code{0200} through @code{0237}, and the remaining bytes are in the -range from @code{0240} through @code{0377}. -@end itemize -@end quotation - -@noindent -In this code, any sequence that starts with a byte in the range -from @code{0240} through @code{0377} is invalid. - -And here is another variant which has the advantage that removing the -last byte or bytes from a valid character can never produce another -valid character. (This property is convenient when you want to search -strings for particular characters.) - -@quotation -The basic sequences consist of - -@itemize @bullet -@item -single bytes with values in the range @code{0} through @code{0177}. - -@item -two-byte sequences in which the first byte is in the range from -@code{0200} through @code{0207}, and the second byte is in the range -from @code{0240} through @code{0377}. - -@item -three-byte sequences in which the first byte is in the range from -@code{0210} through @code{0217}, and the other bytes are in the range -from @code{0240} through @code{0377}. - -@item -four-byte sequences in which the first byte is in the range from -@code{0220} through @code{0227}, and the other bytes are in the range -from @code{0240} through @code{0377}. -@end itemize -@end quotation - -@noindent -The list of invalid sequences for this code is long and not worth -stating in full; examples of invalid sequences include @code{0240} and -@code{0220 0300 065}. - -The number of @emph{possible} multibyte codes is astronomical. But a -given computer system will support at most a few different codes. (One -of these codes may allow for thousands of different characters.) -Another computer system may support a completely different code. The -library facilities described in this chapter are helpful because they -package up the knowledge of the details of a particular computer -system's multibyte code, so your programs need not know them. - -You can use special standard macros to find out the maximum possible -number of bytes in a character in the currently selected multibyte -code with @code{MB_CUR_MAX}, and the maximum for @emph{any} multibyte -code supported on your computer with @code{MB_LEN_MAX}. - -@comment limits.h -@comment ISO -@deftypevr Macro int MB_LEN_MAX -This is the maximum length of a multibyte character for any supported -locale. It is defined in @file{limits.h}. -@pindex limits.h -@end deftypevr - -@comment stdlib.h -@comment ISO -@deftypevr Macro int MB_CUR_MAX -This macro expands into a (possibly non-constant) positive integer -expression that is the maximum number of bytes in a multibyte character -in the current locale. The value is never greater than @code{MB_LEN_MAX}. - -@pindex stdlib.h -@code{MB_CUR_MAX} is defined in @file{stdlib.h}. -@end deftypevr - -Normally, each basic sequence in a particular character code stands for -one character, the same character regardless of context. Some multibyte -character codes have a concept of @dfn{shift state}; certain codes, -called @dfn{shift sequences}, change to a different shift state, and the -meaning of some or all basic sequences varies according to the current -shift state. In fact, the set of basic sequences might even be -different depending on the current shift state. @xref{Shift State}, for -more information on handling this sort of code. - -What happens if you try to pass a string containing multibyte characters -to a function that doesn't know about them? Normally, such a function -treats a string as a sequence of bytes, and interprets certain byte -values specially; all other byte values are ``ordinary''. As long as a -multibyte character doesn't contain any of the special byte values, the -function should pass it through as if it were several ordinary -characters. - -For example, let's figure out what happens if you use multibyte -characters in a file name. The functions such as @code{open} and -@code{unlink} that operate on file names treat the name as a sequence of -byte values, with @samp{/} as the only special value. Any other byte -values are copied, or compared, in sequence, and all byte values are -treated alike. Thus, you may think of the file name as a sequence of -bytes or as a string containing multibyte characters; the same behavior -makes sense equally either way, provided no multibyte character contains -a @samp{/}. - -@node Wide Char Intro, Wide String Conversion, Multibyte Char Intro, Extended Characters -@section Wide Character Introduction - -@dfn{Wide characters} are much simpler than multibyte characters. They -are simply characters with more than eight bits, so that they have room -for more than 256 distinct codes. The wide character data type, -@code{wchar_t}, has a range large enough to hold extended character -codes as well as old-fashioned ASCII codes. - -An advantage of wide characters is that each character is a single data -object, just like ordinary ASCII characters. Wide characters also have -some disadvantages: - -@itemize @bullet -@item -A program must be modified and recompiled in order to use wide -characters at all. - -@item -Files of wide characters cannot be read by programs that expect ordinary -characters. -@end itemize - -Wide character values @code{0} through @code{0177} are always identical -in meaning to the ASCII character codes. The wide character value zero -is often used to terminate a string of wide characters, just as a single -byte with value zero often terminates a string of ordinary characters. - -@comment stddef.h -@comment ISO -@deftp {Data Type} wchar_t -This is the ``wide character'' type, an integer type whose range is -large enough to represent all distinct values in any extended character -set in the supported locales. @xref{Locales}, for more information -about locales. This type is defined in the header file @file{stddef.h}. -@pindex stddef.h -@end deftp - -If your system supports extended characters, then each extended -character has both a wide character code and a corresponding multibyte -basic sequence. - -@cindex code, character -@cindex character code -In this chapter, the term @dfn{code} is used to refer to a single -extended character object to emphasize the distinction from the -@code{char} data type. - -@node Wide String Conversion, Length of Char, Wide Char Intro, Extended Characters -@section Conversion of Extended Strings -@cindex extended strings, converting representations -@cindex converting extended strings - -@pindex stdlib.h -The @code{mbstowcs} function converts a string of multibyte characters -to a wide character array. The @code{wcstombs} function does the -reverse. These functions are declared in the header file -@file{stdlib.h}. - -In most programs, these functions are the only ones you need for -conversion between wide strings and multibyte character strings. But -they have limitations. If your data is not null-terminated or is not -all in core at once, you probably need to use the low-level conversion -functions to convert one character at a time. @xref{Converting One -Char}. - -@comment stdlib.h -@comment ISO -@deftypefun size_t mbstowcs (wchar_t *@var{wstring}, const char *@var{string}, size_t @var{size}) -The @code{mbstowcs} (``multibyte string to wide character string'') -function converts the null-terminated string of multibyte characters -@var{string} to an array of wide character codes, storing not more than -@var{size} wide characters into the array beginning at @var{wstring}. -The terminating null character counts towards the size, so if @var{size} -is less than the actual number of wide characters resulting from -@var{string}, no terminating null character is stored. - -The conversion of characters from @var{string} begins in the initial -shift state. - -If an invalid multibyte character sequence is found, this function -returns a value of @code{-1}. Otherwise, it returns the number of wide -characters stored in the array @var{wstring}. This number does not -include the terminating null character, which is present if the number -is less than @var{size}. - -Here is an example showing how to convert a string of multibyte -characters, allocating enough space for the result. - -@smallexample -wchar_t * -mbstowcs_alloc (const char *string) -@{ - size_t size = strlen (string) + 1; - wchar_t *buf = xmalloc (size * sizeof (wchar_t)); - - size = mbstowcs (buf, string, size); - if (size == (size_t) -1) - return NULL; - buf = xrealloc (buf, (size + 1) * sizeof (wchar_t)); - return buf; -@} -@end smallexample - -@end deftypefun - -@comment stdlib.h -@comment ISO -@deftypefun size_t wcstombs (char *@var{string}, const wchar_t *@var{wstring}, size_t @var{size}) -The @code{wcstombs} (``wide character string to multibyte string'') -function converts the null-terminated wide character array @var{wstring} -into a string containing multibyte characters, storing not more than -@var{size} bytes starting at @var{string}, followed by a terminating -null character if there is room. The conversion of characters begins in -the initial shift state. - -The terminating null character counts towards the size, so if @var{size} -is less than or equal to the number of bytes needed in @var{wstring}, no -terminating null character is stored. - -If a code that does not correspond to a valid multibyte character is -found, this function returns a value of @code{-1}. Otherwise, the -return value is the number of bytes stored in the array @var{string}. -This number does not include the terminating null character, which is -present if the number is less than @var{size}. -@end deftypefun - -@node Length of Char, Converting One Char, Wide String Conversion, Extended Characters -@section Multibyte Character Length -@cindex multibyte character, length of -@cindex length of multibyte character - -This section describes how to scan a string containing multibyte -characters, one character at a time. The difficulty in doing this -is to know how many bytes each character contains. Your program -can use @code{mblen} to find this out. - -@comment stdlib.h -@comment ISO -@deftypefun int mblen (const char *@var{string}, size_t @var{size}) -The @code{mblen} function with a non-null @var{string} argument returns -the number of bytes that make up the multibyte character beginning at -@var{string}, never examining more than @var{size} bytes. (The idea is -to supply for @var{size} the number of bytes of data you have in hand.) - -The return value of @code{mblen} distinguishes three possibilities: the -first @var{size} bytes at @var{string} start with valid multibyte -character, they start with an invalid byte sequence or just part of a -character, or @var{string} points to an empty string (a null character). - -For a valid multibyte character, @code{mblen} returns the number of -bytes in that character (always at least @code{1}, and never more than -@var{size}). For an invalid byte sequence, @code{mblen} returns -@code{-1}. For an empty string, it returns @code{0}. - -If the multibyte character code uses shift characters, then @code{mblen} -maintains and updates a shift state as it scans. If you call -@code{mblen} with a null pointer for @var{string}, that initializes the -shift state to its standard initial value. It also returns nonzero if -the multibyte character code in use actually has a shift state. -@xref{Shift State}. - -@pindex stdlib.h -The function @code{mblen} is declared in @file{stdlib.h}. -@end deftypefun - -@node Converting One Char, Example of Conversion, Length of Char, Extended Characters -@section Conversion of Extended Characters One by One -@cindex extended characters, converting -@cindex converting extended characters - -@pindex stdlib.h -You can convert multibyte characters one at a time to wide characters -with the @code{mbtowc} function. The @code{wctomb} function does the -reverse. These functions are declared in @file{stdlib.h}. - -@comment stdlib.h -@comment ISO -@deftypefun int mbtowc (wchar_t *@var{result}, const char *@var{string}, size_t @var{size}) -The @code{mbtowc} (``multibyte to wide character'') function when called -with non-null @var{string} converts the first multibyte character -beginning at @var{string} to its corresponding wide character code. It -stores the result in @code{*@var{result}}. - -@code{mbtowc} never examines more than @var{size} bytes. (The idea is -to supply for @var{size} the number of bytes of data you have in hand.) - -@code{mbtowc} with non-null @var{string} distinguishes three -possibilities: the first @var{size} bytes at @var{string} start with -valid multibyte character, they start with an invalid byte sequence or -just part of a character, or @var{string} points to an empty string (a -null character). - -For a valid multibyte character, @code{mbtowc} converts it to a wide -character and stores that in @code{*@var{result}}, and returns the -number of bytes in that character (always at least @code{1}, and never -more than @var{size}). - -For an invalid byte sequence, @code{mbtowc} returns @code{-1}. For an -empty string, it returns @code{0}, also storing @code{0} in -@code{*@var{result}}. - -If the multibyte character code uses shift characters, then -@code{mbtowc} maintains and updates a shift state as it scans. If you -call @code{mbtowc} with a null pointer for @var{string}, that -initializes the shift state to its standard initial value. It also -returns nonzero if the multibyte character code in use actually has a -shift state. @xref{Shift State}. -@end deftypefun - -@comment stdlib.h -@comment ISO -@deftypefun int wctomb (char *@var{string}, wchar_t @var{wchar}) -The @code{wctomb} (``wide character to multibyte'') function converts -the wide character code @var{wchar} to its corresponding multibyte -character sequence, and stores the result in bytes starting at -@var{string}. At most @code{MB_CUR_MAX} characters are stored. - -@code{wctomb} with non-null @var{string} distinguishes three -possibilities for @var{wchar}: a valid wide character code (one that can -be translated to a multibyte character), an invalid code, and @code{0}. - -Given a valid code, @code{wctomb} converts it to a multibyte character, -storing the bytes starting at @var{string}. Then it returns the number -of bytes in that character (always at least @code{1}, and never more -than @code{MB_CUR_MAX}). - -If @var{wchar} is an invalid wide character code, @code{wctomb} returns -@code{-1}. If @var{wchar} is @code{0}, it returns @code{0}, also -storing @code{0} in @code{*@var{string}}. - -If the multibyte character code uses shift characters, then -@code{wctomb} maintains and updates a shift state as it scans. If you -call @code{wctomb} with a null pointer for @var{string}, that -initializes the shift state to its standard initial value. It also -returns nonzero if the multibyte character code in use actually has a -shift state. @xref{Shift State}. - -Calling this function with a @var{wchar} argument of zero when -@var{string} is not null has the side-effect of reinitializing the -stored shift state @emph{as well as} storing the multibyte character -@code{0} and returning @code{0}. -@end deftypefun - -@node Example of Conversion, Shift State, Converting One Char, Extended Characters -@section Character-by-Character Conversion Example - -Here is an example that reads multibyte character text from descriptor -@code{input} and writes the corresponding wide characters to descriptor -@code{output}. We need to convert characters one by one for this -example because @code{mbstowcs} is unable to continue past a null -character, and cannot cope with an apparently invalid partial character -by reading more input. - -@smallexample -int -file_mbstowcs (int input, int output) -@{ - char buffer[BUFSIZ + MB_LEN_MAX]; - int filled = 0; - int eof = 0; - - while (!eof) - @{ - int nread; - int nwrite; - char *inp = buffer; - wchar_t outbuf[BUFSIZ]; - wchar_t *outp = outbuf; - - /* @r{Fill up the buffer from the input file.} */ - nread = read (input, buffer + filled, BUFSIZ); - if (nread < 0) - @{ - perror ("read"); - return 0; - @} - /* @r{If we reach end of file, make a note to read no more.} */ - if (nread == 0) - eof = 1; - - /* @r{@code{filled} is now the number of bytes in @code{buffer}.} */ - filled += nread; - - /* @r{Convert those bytes to wide characters--as many as we can.} */ - while (1) - @{ - int thislen = mbtowc (outp, inp, filled); - /* Stop converting at invalid character; - this can mean we have read just the first part - of a valid character. */ - if (thislen == -1) - break; - /* @r{Treat null character like any other,} - @r{but also reset shift state.} */ - if (thislen == 0) @{ - thislen = 1; - mbtowc (NULL, NULL, 0); - @} - /* @r{Advance past this character.} */ - inp += thislen; - filled -= thislen; - outp++; - @} - - /* @r{Write the wide characters we just made.} */ - nwrite = write (output, outbuf, - (outp - outbuf) * sizeof (wchar_t)); - if (nwrite < 0) - @{ - perror ("write"); - return 0; - @} - - /* @r{See if we have a @emph{real} invalid character.} */ - if ((eof && filled > 0) || filled >= MB_CUR_MAX) - @{ - error ("invalid multibyte character"); - return 0; - @} - - /* @r{If any characters must be carried forward,} - @r{put them at the beginning of @code{buffer}.} */ - if (filled > 0) - memcpy (inp, buffer, filled); - @} - @} - - return 1; -@} -@end smallexample - -@node Shift State, , Example of Conversion, Extended Characters -@section Multibyte Codes Using Shift Sequences - -In some multibyte character codes, the @emph{meaning} of any particular -byte sequence is not fixed; it depends on what other sequences have come -earlier in the same string. Typically there are just a few sequences -that can change the meaning of other sequences; these few are called -@dfn{shift sequences} and we say that they set the @dfn{shift state} for -other sequences that follow. - -To illustrate shift state and shift sequences, suppose we decide that -the sequence @code{0200} (just one byte) enters Japanese mode, in which -pairs of bytes in the range from @code{0240} to @code{0377} are single -characters, while @code{0201} enters Latin-1 mode, in which single bytes -in the range from @code{0240} to @code{0377} are characters, and -interpreted according to the ISO Latin-1 character set. This is a -multibyte code which has two alternative shift states (``Japanese mode'' -and ``Latin-1 mode''), and two shift sequences that specify particular -shift states. - -When the multibyte character code in use has shift states, then -@code{mblen}, @code{mbtowc} and @code{wctomb} must maintain and update -the current shift state as they scan the string. To make this work -properly, you must follow these rules: - -@itemize @bullet -@item -Before starting to scan a string, call the function with a null pointer -for the multibyte character address---for example, @code{mblen (NULL, -0)}. This initializes the shift state to its standard initial value. - -@item -Scan the string one character at a time, in order. Do not ``back up'' -and rescan characters already scanned, and do not intersperse the -processing of different strings. -@end itemize - -Here is an example of using @code{mblen} following these rules: - -@smallexample -void -scan_string (char *s) -@{ - int length = strlen (s); - - /* @r{Initialize shift state.} */ - mblen (NULL, 0); - - while (1) - @{ - int thischar = mblen (s, length); - /* @r{Deal with end of string and invalid characters.} */ - if (thischar == 0) - break; - if (thischar == -1) - @{ - error ("invalid multibyte character"); - break; - @} - /* @r{Advance past this character.} */ - s += thischar; - length -= thischar; - @} -@} -@end smallexample - -The functions @code{mblen}, @code{mbtowc} and @code{wctomb} are not -reentrant when using a multibyte code that uses a shift state. However, -no other library functions call these functions, so you don't have to -worry that the shift state will be changed mysteriously. |